1. Field of the Invention
This invention relates to filters for removing high-frequency noise from low-frequency signals, and particularly to filters of the time delay type.
2. Discussion of the Related Art
It is well-recognized by persons skilled in the art that high-frequency noise can blanket a transmission signal, unless it is properly filtered out. A number of methods for filtering signals from the outside world, to clear them of any affecting noise, are to be found described in pertinent literature. Of such methods, the most widely employed are those which either provide an analog low-pass filter or a digital low-pass filter, depending on whether the signal to be filtered is an analog or a digital one.
According to the first of the preferred prior art filtering methods, the input signals are applied, via a resistor, to a first terminal of a capacitor having a second terminal connected to a ground potential. The first terminal is also connected to an output terminal of that RC circuit. Assuming the voltage across the capacitor to be null in the absence of any signal, upon application of a signal, an initial transient would occur whereby the voltage across the capacitor varies with inverse proportion to the time constant fixed by the product of the resistive value of the resistor and the capacitance value of the capacitor, approaching, theoretically after an infinite time, the input signal voltage value. The larger the resistance and capacitance values are, the more slowly the circuit will respond to variations in the input signal.
Therefore, short-duration pulses, as are typical of pulsive noise, don't succeed in affecting the capacitance charge of the capacitor in such a manner to produce significant output variations if compared with the values of the signal they overlap. Such pulses are, therefore, filtered out by the RC circuit.
On the other hand, in the latter of the prior art methods, a fixed rate counter is used as a digital low-pass filter, whereby the state of the input digital signal is controlled to remain the same for at least a predetermined time interval. If the logic level setting of the signal changes during that time interval, the counter is reset and counting is resumed only upon reappearance of the logic level. Accordingly, the output signal of the digital type is determined by the counter end-of-count bit. The count is timed not to exceed the standard duration of the logic level of the signals to be filtered for noise. Only those signals which retain the same logic level throughout the counting period are addressed to the output, and therefore, all signals at a higher frequency will be removed.
Both of the prior methods outlined above are effective, but where a large time constant or a long time count must be provided, the circuits comprising low-pass filters involve significant occupation of integration area, when incorporated into an integrated circuit.
Therefore, an object of the invention is to provide an efficient filtering of digital signals affected by noise, while retaining low integration area requirements of the circuit components compared to known filtering circuits.